This invention relates to evaporation technology. In particular, this invention concerns a method and an apparatus for evaporating a liquid rich in impurities in such a way that no contaminating deposits are formed in the apparatus, especially on the heat transfer surfaces.
In order to avoid contamination, forced circulation evaporators and relatively high flow rates are often employed in the evaporation of contaminating liquids. In the forced circulation evaporation, contamination can also be reduced by placing the heat exchanger at such a low level that the vaporization takes place only in the riser and in the separation tank, after the heat exchanger. If the contamination of the heat surfaces cannot be prevented by means of process engineering, devices provided with continuous mechanical cleaning, such as scraped surface evaporators, are used, or the heat surfaces are cleaned at regular intervals. The same problems are encountered in evaporation crystallization.
It may be possible according to aspects of the invention is to provide a liquid evaporation method, wherein contamination of the heat transfer surfaces is avoided by using a special circu lating fluid by means of which the liquid to be evaporated is heated and conveyed to a va por separation tank. The circulating fluid must have a substantially lower vapor pressure than the liquid to be evaporated and it is not allowed to react chemically with the liquid to be evaporated. Furthermore, the, the solids separating from the liquid to be evaporated must be wetted but not dissolved by the circulating fluid, in order to prevent their sticking to the walls of the apparatus.
It may be possible according to aspects of the invention is to provide a liquid evaporation apparatus, that makes it possible to evaporate a liquid, according to the method of the in vention, by means of a circulating fluid acting as a heat transfer medium. In the evaporator according to the invention, heat transfer surface contamination is also suppressed by mix ing the liquid to be evaporated with the circulating fluid only after the heat exchanger, whereby the heat transfer surface contaminating liquid to be evaporated does not make direct contact with the heat surface.
Preferably, the liquid to be evaporated is substantially insoluble in the circulating fluid. A mechanical mixer or an ejector can be used to ensure that the liquid to be evaporated is mixed with the circulating fluid. By mixing the liquid to be evaporated in the form of small drops with the circulating fluid and by using a turbulent flow in the tube, a contribution is made to the transfer of heat from the circulating fluid to the evaporating droplet. The droplet begins to evaporate when the vapor pressure of the liquid to be evaporated is higher than pressure in the tube. By using a pressure control valve, the vaporization can be adjusted to take place only after the pressure control valve. Since most of the heat required for the evaporation has to be transferred to the droplets of the liquid to be evaporated during vaporization, it is important to dimension the tube located after the pressure control valve in such a way that there is a two-phase flow form favorable to heat transfer in the tube and that the residence time in the tube is long enough. The residence time can be extended by disposing the pressure control valve at a substantially higher level than the separation tank so that the liquid flows downward in the tube in which the vaporization takes place, and the static pressure difference enhances the vaporization, thus compensating for the effect of pressure loss due to flow.
In the separation tank, the vapor is separated from the circulating fluid and conveyed to a condenser. The lower part of the separation tank can be provided with a mass transfer surface (with filling units, for example) in order to remove the biggest possible amount of liquid to be evaporated from the fluid returning into the cycle. The circulating fluid is pumped, together with the solids (crystallized or precipitated) separated from the liquid to be evaporated, to be re-heated in a heat exchanger. A small, continuous flow of circulating fluid can be conveyed out of the system after the pump, in which case a corresponding amount of fresh circulating fluid is fed into the circuit to maintain the solids content of the circulating fluid at a desired maximum level. The solids content of the circulating fluid can also be lowered by means of a separator, filter or another separating device, such as a decanter or settling tank.
The temperature of the circulating fluid in the separation tank and after the heat exchanger is chosen in such a way that most of the liquid to be evaporated vaporizes and the circulating fluid in the separation tank contains only a small amount of liquid to be evaporated, or none at all. The rise in boiling temperatures and the deviation from equilibrium caused by salts have to be considered when choosing the temperatures of the circulating fluid.
The evaporation process according to the invention can be designed to use either pump circulation (forced circulation) or natural circulation. Natural circulation is created by means of vapor bubbles, by arranging the vaporization to occur in a tube rising toward the separation tank.
The evaporation process according to the invention can be designed to be carried out at positive pressure, atmospheric pressure or negative pressure. In terms of contamination and energy consumption, the most preferable evaporation pressure is negative.
Preferably, the method according to the invention can be used for evaporating wastewater saturated or nearly saturated with salts by using, as the circulating fluid, oil or waste oil which is appropriately incinerated after use in order to dispose of the noxious substances.
In order to save energy, the evaporation process can also be designed as a single or multiple stage process, in which case the vapor coming from the previous effect is led to be condensed in the circulating fluid heat exchanger of the following stage. As far the liquid to be evaporated is concerned, the stages are connected in parallel, i.e. each stage is provided with an own supply of evaporation liquid.
The method can be applied to evaporation crystallization by separating the crystals formed in the circulating fluid from the fluid using a centrifuge, a separator or other mechanical separating device, and the crystals thus recovered are washed with a suitable solvent.